1. Field of the Invention
The present invention relates to an aqueous secondary battery and, more particularly, to an aqueous secondary battery in which sodium is dissolved.
2. Description of the Related Art
Aqueous lithium-ion secondary batteries that contain an aqueous solution as an electrolyte solution are conventionally known. In general, aqueous lithium-ion secondary batteries have the following advantages over the problems of non-aqueous lithium-ion secondary batteries. Since aqueous lithium-ion secondary batteries contain no organic solvent in the electrolyte solution, aqueous lithium-ion secondary batteries are basically non-flammable. Furthermore, since aqueous lithium-ion secondary batteries obviate the necessity for dry environments in a manufacturing process, manufacturing costs can be considerably reduced. In addition, since aqueous electrolyte solutions have higher electrical conductivity than non-aqueous electrolyte solutions, aqueous lithium-ion secondary batteries have lower internal resistance than non-aqueous lithium-ion secondary batteries. Contrary to these advantages, since aqueous lithium-ion secondary batteries should be used in such an electric potential range that water is not electrolyzed, aqueous lithium-ion secondary batteries have smaller electromotive forces than non-aqueous lithium-ion secondary batteries. Thus, aqueous lithium-ion secondary batteries ensure a high degree of safety, low cost, and low internal resistance at the expense of high voltage and high energy density.
Example of negative-electrode active materials for aqueous lithium-ion secondary batteries include Fe oxides (see, for example, Japanese Unexamined Patent Application Publication No. 2000-340256), Fe polyanion compounds (see Japanese Unexamined Patent Application Publication No. 2002-110221), and lithium vanadium oxides (see Japanese Unexamined Patent Application Publications No. 2001-102086, No. 2000-77073, and No. 2003-17057).
A capacitor recently proposed contains a Mn-based sodium ion insertion/extraction material, such as Na4Mn9O18 (see, for example, L. Athouel et al., J. Phys. Chem. C, (2008) 112, 7270-7277), NaMnO2 (see Q. T. Qu et al., J. Power Sources, 194 (2009) 1222-1225), or MnO2 (see J. F. Whitacre et al., Electrochem. Commun., 12 (2010) 463-466), as a positive electrode and an activated carbon as a negative electrode. The activated carbon utilizes electric double layer capacitance.